RFR: Align plabs on card boundaries

[William Kemper]

On Wed, 27 Oct 2021 19:22:27 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> If plabs are not aligned on remembered set card boundaries, then we need synchronization locks to register the promotion objects allocated by each thread.  We do not currently have these synchronization locks and do not want to add them.  Instead, we are enforcing that all of the objects that reside within a particular remembered set card are allocated by the same thread.

Looks good  to me.

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Marked as reviewed by wkemper (Committer).

PR: https://git.openjdk.java.net/shenandoah/pull/95